1. Field of the Invention
The present invention relates to an optical information reproducing apparatus for binarizing a reproduction signal from an optical information recording medium.
2. Related Background Art
In recent years, optical recording media such as optical discs and the like are rapidly being advanced as types of memory of high density and large capacity, including a rewritable magnetooptical disc as well as a read-only CD and a write-once type disc. Particularly, the magnetooptical disc is proving useful as an external memory of high density and large capacity for use with a computer. In association with the large capacity of the disc apparatus, as means for increasing the memory capacity by effectively using the possible recording density on a disc, hitherto, there has been used what is called a mark position recording, wherein each bit of one polarity (say, "1") is recorded as a pit of a single length. In recent years, however, what is called mark edge, in which a higher density is realized by providing information for edges on both sides of a pit, is becoming the main stream approach.
In such an optical disc, since the information has been recorded at a high density, it is required to accurately read out the recorded information upon reproduction. In the approach used hitherto, after magnetooptical signals read out from two photosensors are amplified by a preamplifier, a difference signal between outputs of the two sensors is formed, the peak value and the bottom value of the difference signal are further detected, and the difference signal is sliced at the center value of the peak and bottom values and is binarized, thereby reproducing the magnetooptical signal. However, in a case where the recording conditions are changed due to the following causes or the apparatus is influenced from the outside, the level of the reproduction signal fluctuates, so that there is a case where the magnetooptical signal cannot be accurately reproduced:
(1) In the magnetooptical disc apparatus, the writing level of the magnetooptical signal differs for every apparatus. PA1 (2) In the magnetooptical disc apparatus, the writing level of the magnetooptical signal differs for every disc, and the writing level also differs in dependence on the position on the disc. PA1 (3) In a case where dust is deposited on the magnetooptical disc or the disc becomes dirty, the reproduction signal level complicatedly fluctuates. PA1 (4) Waveform interference occurs due to the high density recording. PA1 (5) The apparatus is influenced by the DC component in dependence on a modulating system (for example, 1-7 modulation).
FIGS. 1A to 1C are waveform diagrams showing changes in reproduction signal when a writing level of a magnetooptical signal is changed. FIG. 1A shows the reproduction signal written at an excessive level. FIG. 1B shows the reproduction signal written at a proper level. FIG. 1C shows the reproduction signal written at too low a level. As will be obviously understood from those diagrams, in the case where the signal was written at the excessive or too low level, the slice level to be set at the center value of the peak and bottom values is improper, so that the pulse width that ought to be reproduced, cannot be reproduced and output.
FIG. 2 shows a problem which is caused due to influence by the DC component. There is a case where in 1-7 modulation or the like, a reproduction waveform as shown in the diagram appears in dependence on a modulation pattern. In such a case, although no problem occurs in block B, in case of block A, if the center value of the peak and bottom values is set to a slice level, a correct pulse width cannot be reproduced.
As a measure for solving the above problems, an optical information reproducing apparatus shown in FIG. 3 has been proposed. In FIG. 3, reference numeral 51 denotes a peak-hold circuit; 52 a bottom-hold circuit; 53 a center level detection circuit; 54 a comparator; 55 a leading edge detection circuit; 56 a trailing edge detection circuit; 57 and 59 phase locked loop (PLL) circuit; 60 and 61 synchronous circuits; 62 and 63 first-in first-out memory circuits (FiFo circuits); and 64 a data composition circuit.
In the apparatus of FIG. 3, a center value which is detected by the center level detection circuit 53 from outputs of the peak-hold circuit 51 and bottom-hold circuit 52 is set to a slice level and a binarization is coarsely performed by the comparator 54. Different PLLs are applied at the leading and trailing edges of a binary signal and the resultant reproduction data is inputted to the FiFo circuits 62 and 63. Subsequently, by reading out the reproduction data by a clock which is generated on the controller side, a fluctuation in recording power, a fluctuation in amplitude, and an influence by the DC component are reduced.
The conventional apparatus as shown in FIG. 3, however, because a plurality of PLL circuits and FiFo circuits are necessary, is disadvantageous in terms of the costs. There is also a problem in that the use of a plurality of PLL circuits is disadvantageous from the viewpoint of noise or the like in the case of realizing an LSI, or the like.
As another binarizing method different from the apparatus of FIG. 3, as disclosed in JP-A-5-40978, the following method is also proposed. Namely, after the difference signal of the signals from two sensors is binarized on the basis of a predetermined level, a clock signal is extracted from the binary signal, and the slice level is controlled so that the average value of the binary signal synchronized by the clock or the pulse width is equalized to that of the binarizing means.
However, in order to obtain a voltage corresponding to a difference of pulse widths (duty deviation) by differential detection after averaging operation, there was a problem in that it was hard to assure accuracy, for the following reasons.
(1) The difference of an amplitude of a binary signal to be compared and that of the synchronized binary signal would impart an extremely large error in comparison to the voltage corresponding to the difference of the pulse widths desired to be detected. For this reason, it is necessary to make the difference of the amplitudes extremely small.
(2) It is necessary to set a time constant so as not to saturate an averaging circuit with respect to an input signal. However, under condition of such a time constant, the voltage to be detected as a difference of pulse widths is extremely small. This is because the difference of the pulse widths desired to be detected is extremely small in comparison to the pulse width.
In the case of averaging the signals and asynchronously using the average output for the slice level control as mentioned above, a time constant for the averaging process must be large so as not to cause an oscillation phenomenon. There is consequently a problem in that a time delay occurs in the detection of a duty deviation and the automatic slice level control cannot be performed at a high speed.
The present invention is made in consideration of the above problems and it is an object of the invention to provide an optical information reproducing apparatus which can reproduce an accurate magnetooptical signal and can perform a binarization of a high response speed and a high precision irrespective of complicated fluctuations in level of a reproduction signal and a duty due to a change in writing level or the like of the magnetooptical signal.